1. Field of the Invention
The invention relates to a process for the production of catalytic converters for purifying exhaust gases, and more particularly to a method for producing the catalytic converter utilizing a pre-formed rectangular metal sheet exhibiting a substantially uniform curvature.
2. Description of the Related Art
As is well known, the purification of exhaust gases from internal combustion engines, particularly in motor vehicles, is generally achieved by an exhaust gas purification system in which a ceramic element having a honeycomb cell structure acts a catalyst carrier. More precisely, this honeycomb cell structure is covered with a catalyst which contains a precious metal which functions, in the presence of O.sub.2, to convert noxious components of the exhaust gas, such as HC and CO, to CO.sub.2 and H.sub.2 O. The honeycomb cell structure is housed within a gas-tight, sheet metal or cast-metal heat resistant housing or can.
Honeycomb structures currently employed are typically comprised of a ceramic material such as cordierite; a brittle material exhibiting limited mechanical strength. For this reason, catalytic converters in use today, typically include a supporting mat which is wrapped around the periphery of the honeycomb. This resilient material, which distributes any compressive forces uniformly on the ceramic, typically expands as the temperature increases. This being the case, the compressive supporting pressure on the honeycomb therefore increases at elevated temperatures, and in some degree compensates for the thermal expansion of the outer metal shell. Since the metal shell expands more than the enclosed ceramic honeycomb, this mat expansion with temperature rise prevents the honeycomb from becoming loose in the can shell.
There are known to the art various methods of fabricating catalytic as described above, including inserting tight-fitting mat-wrapped honeycombs into tubular shells (see, for example U.S. Pat. No. 4,093,423 (Neumann) ), as well as utilizing two metal shell halves which are closed around a mat-wrapped honeycomb and thereafter welded together; see for example U.S. Pat. No. 5,273,724 (Bos). Another such method of fabrication, commonly referred to as the "tourniquet wrap" method involves forming a rectangular flat sheet metal piece into a cylindrical body having a lap joint. A mat-wrapped honeycomb is loosely inserted into the cylindrical metal can and the combined assembly is pulled together to form the desired mat compression. Thereafter, the lap joint is welded together thereby holding the can at the desired compression while at the same time preventing gas leakage; see for Example U.S. Pat No. 5,082,479 (Miller).
The aforementioned forming of a rectangular flat sheet metal piece into a cylindrical body is usually accomplished using a three-roller sheet metal rolling device and technique. An inherent deficiency of these pre-rolled metal bodies is that the end portions of the cylindrical metal body do not have the same curvature as the middle portion of the cylindrical sheet. An outcome of using these cylindrical metal bodies, exhibiting non-uniform curvature, is that irregular gaps are formed between the honeycomb structure and the metal body or can. While the supporting mat acts to alleviate some of this non-uniformity, there is imposed upon the honeycomb non-uniform compressive loading. In those areas where the gap is the smallest, point loading leads to localized compressive failure of the honeycomb structure, i.e., crushing of the brittle honeycomb structure.